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Re: Preprocessor and "empty" substitutions

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Andrey Tarasevich <andreytarasevich@hotmail.com> writes:

> Consider the following two simple tests
>
>   #define OUTPUT(x) printf("%d\n", x)
>
>   #define EMPTY
>   #define TEST(x) OUTPUT EMPTY (x)
>
>   int main(int argc, char **argv)
>   {
>     int i = 42;
>     TEST(i);
>   }

Here is the sequence (I'll put tokens in <> to make them stand out):

A function-like macro is detected because <TEST> is followed by <(>.
Arguments are collected.  We get <i>.  The token list

  <OUTPUT> <EMPTY> <(> <x> <)>

has parameter <x> replaced by <i> to get:

  <OUTPUT> <EMPTY> <(> <i> <)>

Now this list is scanned for further replacement, but the macro name
token <OUTPUT> is not followed by a <(> token so no function-like macro
invocation is seen.

<EMPTY> is seen as an object-like macro and is replaced by nothing, but
the scan does not go back to see if this makes any difference to the
decision not to recognise an invocation of a function-like macro.  No
further tokens can be replaced and, importantly, the resulting list:

  <OUTPUT> <(> <i> <)>
  
is /not/ scanned again.

<cut>
>   #define OUTPUT(x) printf("%d\n", x)
>
>   #define TEST(x, y) OUTPUT y (x)
>
>   int main(int argc, char **argv)
>   {
>     int i = 42;
>     TEST(i, );
>   }

In this case, the token list that results from parameter substitution is:

  <OUTPUT> <(> <x> <)>

and, since the list /is/ re-scanned after parameter substitution, the
invocation of the function-like macro is seen and replaced.

<cut>
> So, what exactly happens in the first case? What does 'EMPTY' leave
> behind after substitution? Which specific wording the the standard
> says that empty parameter values are truly "empty", while empty macros
> are not?

It's not really about what's empty and what isn't.  I hope the above
explains it though.

-- 
Ben.

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Thread

Preprocessor and "empty" substitutions Andrey Tarasevich <andreytarasevich@hotmail.com> - 2019-10-07 12:03 -0700
  Re: Preprocessor and "empty" substitutions Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-10-07 21:05 +0100
    Re: Preprocessor and "empty" substitutions Andrey Tarasevich <andreytarasevich@hotmail.com> - 2019-10-07 15:16 -0700
      Re: Preprocessor and "empty" substitutions Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-10-08 01:41 +0100
        Re: Preprocessor and "empty" substitutions Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-10-09 01:58 +0100
        Re: Preprocessor and "empty" substitutions Andrey Tarasevich <andreytarasevich@hotmail.com> - 2019-10-08 11:41 -0700
          Re: Preprocessor and "empty" substitutions Tim Rentsch <tr.17687@z991.linuxsc.com> - 2019-10-09 06:34 -0700

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